Impact-resistant and penetration-resistant materials find uses in many applications such as sports equipment, safety garments, and personal body armor.
Various fiber-reinforced constructions are known for use in impact-resistant, ballistic-resistant and penetration-resistant articles such as helmets, panels, and vests. These articles display varying degrees of resistance to penetration by impact from projectiles or knives, and have varying degrees of effectiveness per unit of weight.
For example, a measure of the ballistic-resistance efficiency is the energy removed from a projectile per unit of the target's areal density. This is known as the Specific Energy Absorption, abbreviated as “SEA”, and having units of Joules per kg/m2 or J-m2/kg. The SEA of a fibrous construction is known to generally increase with increasing strength, tensile modulus and energy-to-break of the constituent fibers. However, other factors, such as the shape of the fibrous reinforcement, may come into play. U.S. Pat. No. 4,623,574 presents a comparison between the ballistic effectiveness of a composite constructed with a ribbon-shaped reinforcement versus one using a multi-filament yarn: both of ultra-high molecular weight polyethylene in an elastomeric matrix that substantially coated each of the individual fibers. The fiber had a higher tenacity than the ribbon: 30 grams/denier (g/d) (2.58 GPa) versus 23.6 g/d (2.03 GPa). Nevertheless, the SEA of the composite constructed with the ribbon was somewhat higher than the SEA of the composite constructed with the yarn.
One example of the preparation of UHMWPE melt-blown film is provided by Takashi Nakahara et al., “Ultra High Molecular Weight Polyethylene Blown Film Process,” ANTEC 2005, 178-181 (2005). Film made by this process was slit and drawn to make a high strength tape. The tenacity of the tape made from the stretched blown film was less than 20 g/d (1.72 GPa).
U.S. Pat. Nos. 5,091,133; 5,578,373; 6,951,685; and 7,740,779 disclose polyethylene powder being compressed at an elevated temperature to bond the particles into a continuous sheet that is then further compressed and stretched. U.S. Pat. No. 5,091,133 describes a fiber made by this latter process having a tensile strength of 3.4 GPa. Polyethylene tapes so produced are commercially available under the trademark TENSYLON® by BAE Systems. The highest tenacity reported on the TENSYLON® web site is 19.5 g/d (tensile strength of 1.67 GPa).
An example describing the preparation of a UHMWPE composite made from Spectra® fiber is provided by Yachin Cohen et al., “A Novel Composite Based on Ultra-High-Molecular-Weight Polyethylene”, Composites Science and Technology, 57, 1149-1154 (1997). Spectra® fibers under tension were treated with a solvent in order to swell the fiber surface and promote adhesion between fibers while forming a prepreg. The yarn prepreg was then wound on a plate to produce unidirectional layers which were then pressed and heated and the solvent removed to yield a composite sheet material containing UHMWPE fiber in a re-crystallized UHMWPE matrix formed from the previously dissolved fiber surfaces. The researchers state that the unique properties of UHMWPE make it a desirable candidate for a matrix material to be used with UHMWPE fibers; however, this had not been possible before the use of their solvent-based process for several reasons: (1) The difference in the melting temperatures of the oriented UHMWPE fibers and the un-oriented UHMWPE matrix is too small, (2) The extremely high melt viscosity of UHMWPE results in negligible melt flow in molding processes for formation of the composite materials, (3) The relatively poor adhesion of untreated UHMWPE fibers to UHMWPE matrix.
U.S. Pat. No. 5,135,804 describes high strength plaques made by heating and pressing unidirectionally aligned gel-spun polyethylene fibers without any solvent or resin treatment of the fibers before pressing. Example plaques were formed by winding fibers around a 3-inch square metal plate and then pressing the assembly for several minutes in a heated press. The hot-pressed UHMWPE plaques were substantially free of voids and essentially transparent.
U.S. Pat. No. 5,628,946 describes a homogeneous polymeric monolith made from thermoplastic polymer fibers which were first pressed into contact with each other at an elevated temperature adequate to selectively melt a portion of the polymer fibers and then pressed at a second higher pressure at the elevated temperature to further consolidate the material. An example of a monolithic sheet of dimensions 3 mm by 55 mm by 55 mm made from Spectra® fiber is given, in which a unidirectionally aligned bundle of the fibers was pressed in a mold at 152° C. for 10 minutes at a first pressure and 30 seconds at a higher pressure. It is stated that a DSC trace of the pressed sheet showed around 350 of a “second phase” formed by melting of the original fiber.